An alternator is an electrical generating machine that produces alternating voltages and currents, the frequency of which is proportional to the speed at which the inductor or rotor of the alternator is driven. The rotor induces an alternating current in the stator windings of the machine. At the output of the stator windings, diodes are provided to rectify the alternating current so that a unidirectional (ideally a d.c.) voltage is available in the vehicle.
An alternator requires voltage regulation such as to produce an electromotive force which will be substantially constant, regardless of the number of circuits demanding an output from the machine at any given moment, and regardless of the speed of the engine of the vehicle in order to ensure correct charging of the battery of the vehicle.
The principle of regulation which is currently in general use is illustrated in FIG. 1 of the accompanying drawings, to which reference is invited. At the output of the diodes which constitute the rectifier bridge (not shown) there is connected a regulator in which the rectified alternator voltage Ub+ is used so as to enable the excitation current Ii in the inductor winding I to be regulated by means of a power control stage Tr of the regulator. This power stage Tr is indicated in FIG. 1 in the form of a transistor, the emitter of which is connected to earth, with its collector being connected to one end of the inductor winding 1. The other end of the inductor winding is connected to the alternator output which is itself connected to the battery.
The voltage Ub+, after rectification, still displays a waveform of substantially varying amplitude, which is generated in particular by virtue of the switching of the diodes in the rectifier bridge and by parasitic effects emitted by the various devices constituting the load.
In order to overcome this drawback, the regulator also includes a filter F and a comparator C. The voltage Ub+ is filtered through the filter F, the output of which is connected to a first input of the comparator C, which receives a voltage reference Vref on its second input. The output of the comparator C is connected to the base of the transistor Tr. If the voltage Ub+ is smaller than the reference voltage Vref, a current Ii is applied on the inductor by the battery via the filter F and the comparator C. On the other hand, if Ub+ is greater than Vref, the transistor Tr becomes non-conducting, and no current is supplied to the inductor I, that is to say the excitation current is modulated either completely or not at all.
A recirculation diode D which is connected in series, back to back with the power stage Tr, ensures that when the latter is non-conducting, the excitation current Ii is maintained in the inductor winding I between two transitional states of the power stage Tr. Thus, the voltage applied to the load network is regulated after filtering by controlling the excitation of the generating unit comprising the alternator itself and its rectifier bridge. The generator output voltage Ub+ thus has a regulating waveform which mirrors that of the excitation current, with its amplitude varying as a function of the excitation of the alternator. Thus, this voltage is a cyclic voltage having a period T, each cycle of which comprises an alternate period T1, in the course of which the voltage is applied to the inductor, and an alternate period T2, in the course of which no voltage is applied to the inductor. The ratio T1/T2 is called the cyclic ratio, and it is desirable that this cyclic ratio should vary as slowly as possible.
The illustration of the regulating principle that has just been given is of course very sketchy, and in fact, a regulating circuit may use a large number of transistors. This has led to the regulating circuits being made in the form of integrated circuits. Such an integrated regulating circuit may also be a multi-functional circuit, that is to say it not only performs the regulation function itself, but also at least the function of indicating the state of battery charge and possibly also the tension in the belt by which the alternator is coupled to the engine of the vehicle. However, circuits of this kind remain somewhat restricted as to their structure and their mode of operation, and their design and manufacture are both lengthy and costly.
In addition, in conventional regulators, the response of the comparator is of the "all or nothing" kind, that is to say it indicates whether the excitation current should or should not be supplied, but it offers no quantitative values concerned with large variations in the cyclic ratio. These variations are even larger according to whether the comparator C acts with a greater or smaller time delay due to the filter F.
The need to accommodate a number of different parameters dictated by the operation of the alternator, together with the requirements of manufacturers to provide larger and larger amounts of power, lead to a need for careful control of the actual operation of the generator, good adaptation to the system comprising the control circuits of the vehicle, and high operational stability.
In order to overcome these problems, it has already been attempted to use digital techniques by means of a microprocessor. Even though such techniques enable the computation of a theoretical value of the excitation voltage to be carried out as a function of external parameters, such as state of charge of the battery, battery temperature, running state of the engine, belt tension, and so on, their use has still not enabled complete regulation to be achieved. This is because of the difficulty of attenuating varying amplitudes found in the excitation voltage, so that it can be measured sufficiently accurately and reliably.